The Hohonu Dyke Swarm and French Creek Granite represent contemporaneous and cogenetic alkaline magmatism generated during crustal extension in the Western Province of New Zealand. The age of 82 Ma for French Creek Granite coincides with the oldest oceanic crust in the Tasman Sea and suggests emplacement during the separation of New Zealand and Australia. The French Creek Granite is a composite A-type granitoid, dominated by a subsolvus biotite syenogranite with high silica, low CaO, MgO, Cr, Ni, V and Sr and elevated high-field-strength elements (Zr, Nb, Ga, Y). Subordinate varieties of French Creek Granite include a hypersolvus alkali amphibole monzogranite and a quartz-alkali feldspar syenite. Spatially associated rhyolitic dykes are considered to represent hypabyssal equivalents of French Creek Granite. The Hohonu Dyke Swarm represents mafic magmatism which preceded, overlapped with, and followed emplacement of French Creek Granite. Lamprophyric and doleritic varieties dominate the swarm, with rare phonolite dykes also present. Geochemical compositions of French Creek Granite indicate it is an A1-subtype granitoid and suggest derivation by fractionation of a mantle-derived melt with oceanic island basalt - like characteristics. The hypothesis that the French Creek Granite represents fractionation of a Hohonu Dyke Swarm composition, or a mantle melt derived from the same source, is tested. Major- and trace-element data are compatible with derivation of the French Creek Granite by fractionation of amphibole, clinopyroxene and plagioclase from mafic magmas, followed by fractionation of alkali and plagioclase feldspar at more felsic compositions. Although some variants of the French Creek Granite have Sr and Nd isotopic compositions overlapping those of the Hohonu Dyke Swarm, most of the French Creek Granite is more radiogenic than the Hohonu Dyke Swarm, indicating the involvement of a radiogenic crustal component. Assimilation-fractional crystallisation modelling suggests isotopic compositions of French Creek Granite are consistent with extreme fractionation of Hohonu Dyke Swarm magmas with minor assimilation of the Greenland Group metasediments.